(define (all-labelled-subtrees t)
  (match t
    [`(,label . ,children) (cond
                             [(and (not (list? label)) (empty? children)) '()]
                             [(list? label) (cons label (all-labelled-subtrees t))]
                             [else (all-labelled-subtrees t)])]
    [_ (error "no u")]))


(define (helper t)
  (list (all-labelled-subtrees `(,t))))

(define t '(1 2 3 (4 5 (6 7 (9)))))